In underground drilling, such as gas, oil, or geothermal drilling, a bore is drilled through a formation deep in the earth. Such bores are formed by connecting a drill bit to sections of pipe, referred to as “drill pipe,” so as to form an assembly commonly referred to as a “drill string” that is suspended from a rig at the surface and that extends down to the bottom of the bore. The drill bit is rotated so that it advances into the earth, thereby forming the bore. In rotary drilling, the drill bit is rotated by rotating the drill string at the surface. In directional drilling, the drill bit is rotated by a down hole mud motor coupled to the drill bit; the remainder of the drill string is not rotated during drilling. In a steerable drill string, the mud motor is bent at a slight angle to the centerline of the drill bit so as to create a side force that directs the path of the drill bit away from a straight line. In any event, in order to lubricate the drill bit and flush cuttings from its path, piston operated pumps on the surface pump a high pressure fluid, referred to as “drilling mud,” through an internal passage in the drill string and out through the drill bit. The drilling mud then flows to the surface through the annular passage formed between the drill string and the surface of the bore.
Depending on the drilling operation, the pressure of the drilling mud flowing through the drill string will typically be between 0 and 25,000 psi. In addition, there is a large pressure drop at the drill bit so that the pressure of the drilling mud flowing outside the drill string is considerably less than that flowing inside the drill string. Thus, the components within the drill string are subject to large pressure forces. In addition, the components of the drill string are also subjected to wear and abrasion from drilling mud, as well as the vibration of the drill string.
Throughout the drilling operation, a drill bit may be subject to various loads that act on the drill string. The fundamental loads acting on the drill string are: axial tension, torsion, bending, pressure and temperature. All of these loads result in strain being applied to the drill string. These loads may be static or dynamic and fluctuate during the drilling process. The axial tension loads are due to applying a drilling weight to the drill bit. This is normally referred to “weight-on-bit” or WOB. The actual amount of weight-on-bit depends on the entire weight of the drillstring and the amount tensile load applied at the rig. This is typically referred to as “hook load”. Secondary loads that effect the weight applied to the bit are hydrostatic loads and friction loads. The hydrostatic loads depend on the depth at the bit and the density of the drilling fluid. There are also friction loads along the length of the drillstring due to contact between the drillstring and the borehole. Torsion loads are applied to the drill string to provide adequate cutting torque at the bit. Bit torque may is applied by two means; by a motor at the rig that rotates the drill string that in turn rotates the bit, and/or by a down hole motor that rotates the a drive shaft just above the bit. Drill string bending comes from the well path curvature, sagging of the drill string due to gravitational forces and buckling. Typically, as the well is drill deeper the temperature increases. However, there may be temperature variations within different formations. The annulus temperature and the temperature within the fluid within the bore of the drill string may be slightly different. Bore temperatures tend to be slightly cooler than the annulus temperatures.
The loads applied to the drill string strain the drill string material. Strain gauges can be used to measure these loads. The strain gauges are positioned on a drill collar such that the are subjected to certain strains. The gauges are connected in a bridge arrangement such that the measure the desired load while eliminating the effects of other loads. For example, the gauges can be mounted on a drill collar and connected to one another such that the bridge measures only the bending load, subtracting out tensile and torque loads. Pressure and temperature stains can not be cancelled out. Therefore, these loads and measured and a correction factor is applied to the measurement. The measurements may be recorded down hole and/or transmitted to the surface by mud pulse or wired pipe.
U.S. Pat. No. 6,547,016, entitled “Apparatus for Measuring Weight and Torque on a Drill Bit Operating in a Well,” hereby incorporated by reference in its entirety, provides methods for calculating weight on bit and torque on bit in such a way that the effects of bending do not effect the calculations, but does not include any manner for measuring the effects of bending.